This invention relates in general to flow control, and more specifically to a novel method and apparatus for regulating the flow of an intravenous fluid to a patient through a deformable tube.
Intravenous (IV) therapy is widely used in hospital care. Approximately 40% of all hospitalized patients receive a solution by IV administration. Intravenous administration has several important medical advantages: providing access to the patient's blood stream for the direct injection of medications; facilitating maintenance of a balanced electrolyte and fluid level; providing a means for controlled delivery of drugs with respect to both dose and timing; and delivering nutritional supplements.
Traditionally, an IV set up consists of a supply bottle or bag, a disposable administration set consisting of polyvinylchloride (PVC) plastic tubing, a drop chamber to allow fluid rate to be set by counting drops and converting to volume rate of flow, a roller or screw clamp that squeezes the tubing to control the size of the internal flow path and thereby controls the flow rate, a Y-site for injection of medications, and a filter for the fluid to be administered. In this set up, fluid flow is driven by gravity, and is a function of hydrostatic pressure head and the pressure drop across the clamp. The hydrostatic pressure is determined by the height of the fluid level above the heart of the patient minus venous pressure. Pressure drop is controlled largely by the cross-sectional area of the flow path restricted by the clamp. Flow rate is set by adjusting the clamp (e.g. by rotating the roller) while counting drops and converting to flow rate using the conversion formula provided by the manufacturer of the intravenous administration set. In the conventional prior art system the flow regulator reduces the flow area at a control site. The clamp can reduce the flow area sufficiently to stop the flow completely, but in all prior art clamps to reopen the flow path the direction of clamping is reversed which ordinarily reduces the clamping force.
A well recognized problem of gravity driven administration sets is inconsistency of the flow rate over time. Numerous clinical studies have been conducted to determine the source of this problem. For example, E. W. Clarke et al. reported such a study in "Impairment of Flow in Routine Gravity-Fed Intravenous Infusions to Surgical Patients" in Clinical Science, 57, 515-520 (1979), in which they concluded that the decrease in flow rate was due to the gradual deformation of the plastic tubing under the regulator clamp. Other studies include "Behavior of Standard, Gravity-fed Administration Sets Used for Intravenous Infusion" by F. C. Flash and T. D. White in British Medical Journal, 3:439-443 (1974) and "Electronic Flow Control and Roller Clamp Control in Intravenous Therapy" by B. A. Bivens et al in Arch. Surg. 70-72 (1980).
This problem is particularly complex and troublesome for viscoelastic plastic materials such as PVC. When the tube is clamped from its initial cylindrical shape, stresses induced by the clamping cause a gradual deformation of the tubing over time known as "cold flow" or "creep". These changes in the tubing result in corresponding changes in the tube flow path dimensions and therefore the fluid flow rate through the flow path. Flow rate is very sensitive to the size of the orifice, varying with the fourth power of the radius and the second power of the cross-sectional area. (Also, the clamp itself can deform over time.) However, since PVC has a significant cost advantage over alternative materials, it continues to be the preferred material for IV administration tubing. The usual solution is to monitor the flow rate and make periodic manual adjustments in the clamp to compensate for the creep and thereby to stabilize the flow rate. This solution, however, is subject to human error, risks substantial variations in flow rates between adjustments, and requires the time and regular attention of a nurse or other health care person.
Examples of prior art clamps are disclosed by U.S. Pat. No. 3,685,787 (now Re 31,584); U.S. Pat. No. 4,013,263; and U.S. Pat. No. 4,047,694 to Adelberg, U.S. Pat. No. 3,802,463 to Dabney, and U.S. Pat. No. 4,434,963 to Russell et al. The '787 patent to Adelberg discloses a roller clamp where a roller is mounted on a shaft that is journaled for movement within two parallel, longitudinally extending grooves. Opposite the roller is a groove, typically V-shaped, with longitudinally varying dimensions that in combination with the position of the roller controls the minimum dimensions of the flow lumen. There is no arrangement for securing the roller reliably in one position along the tubing. The walls of the tube alongside the V-shaped groove are in contact, but only slightly compressed; the purpose is simply to confine flow to the orifice defined by the groove and location of the roller. While this patent attempts to resolve the problem of cold flow of the plastic material at the clamped section, in practice it has not done so reliably.
The '263 and '694 patents teach modified embodiments of the roller clamp of the '787 patent which address the same problems. In the '263 patent, a plurality of ridged elements are formed on the surface of the roller clamp which act locally to pinch or grip the tubing at discrete intervals. In the '694 patent, the tubing is clamped over a variable cross-section longitudinal channel and the edges of the roller wheel are undercut to form stepped shoulders or recesses between the wheel and the side walls of the clamp body so that the tubing can migrate into these areas when there are longitudinal forces acting on the tubing. (It is significant to note that this clamp is designed to push tubing material away from the flow path.) Due to this design which compresses a relatively broad section across the entire width of the tubing, a larger force is required to achieve the same degree of compression than with a narrow or pointed means of compression. This increased force causes creep not only in the tubing, but in the plastic clamp itself, leading to changes in the position of the roller relative to the tubing over time. More generally, this problem is characteristic of all conventional roller clamps now on the market since the roller presents a large surface area in contact with the tubing along a portion of its circumference; thus a relatively large force is required to produce a desired stress in the tubing.
Another problem with this type of clamp is that after cold flow away from the clamped region, the tubing has thinned and the clamping roller is free to "fall" onto the reduced thickness rubing. This changes the applied force, and the flow rate. Still another problem with conventional roller clamps of the Adelberg type is that if large comressive forces are applied to the tubing, the stress in the tubing walls is substantial and causes creep. In particular, when a round tubing is compressed under a roller against a hollow channel with large compressive forces, the upper tubing wall is under sufficient stress to bow away from the roller toward the flow path, as shown in FIGS. 3b, 7 and 8 of Adelberg Re 31,584. In this situation, the geometry of the flow path is not time stable and the stresses acting within the tubing walls, whether compressive, tensile or shear, are not in equilibrium until the material cold flows to relieve the stresses.
The '463 patent discloses a roller clamp that compresses a longitudinal section of the plastic tubing, leaving the sides uncompressed, thereby forming a pair of side orifices through which the fluid flows. While Dabney attempts to deal with the same general problem as the present invention, in the central compressed section of the tubing, Dabney, like Adelberg, brings the inner walls into contact only to stop the flow (which Dabney terms a "slight compression") and to define a flow lumen where the inner walls are not in contact. Compressing a tubing from an initially circular configuration to the double lumen configuration of Dabney introduces significant internal stresses that over time cause creep and variations in the fluid flow rate through the two side lumens.
The '963 Russell et al. patent discloses a simple slide clamp where the tubing is received in an internal slot with a longitudinally varying gap. Sliding the tubing toward the narrower end of the gap restricts and then closes off a flow through the tubing. Such devices are used as "on-off" clamps, not to produce a variable flow rate and certainly not to produce a flow at a time-stable rate.
In practice none of the prior art devices has provided a reliably consistent, well-controlled, time-stable administration of fluids to a patient using a standard IV administration tubing set without repeated manual readjustment. To date the standard industry solution to the accuracy problem has been in-line flow valves and electronic flow control devices. These devices provide the desired control but they are much more expensive than external clamps of the type shown in the Adelerg or Dabney patents. Thus their use is generally limited to critical care and specialty applications for medical or surgical patients. Heretofore there has been no low-cost IV administration set suitable for routine patient care that also delivers a fluid at a well-controlled rate over an extended period of time.
It is therefore an object of the present invention to provide a method and apparatus that produce a fluid flow rate that is exceptionally stable over an extended period of time and therefore does not require frequent manual readjustment.
It is further object of the present invention to provide a method for controlling fluid flow and a flow control device wherein a stable flow rate is achieved within minutes after initial adjustment and remains generally stable, with less than a 10% change from the initial flow rate.
It is another object of the present invention to provide an inexpensive, disposable, and easily manfactured flow control device and method for using the same.
It is yet another object of the present invention to provide a method for regulating intravenous fluid flow which does not require electronic controllers or in-line devices that come in contact with the fluid.
A further object of this invention is to provide a flow control method and apparatus that provides the foregoing advantages even when the tubing is PVC.
Still another object is to provide a flow control device with the foregoing advantages that requires relatively low force levels to achieve the desired flow control.